In the event of a motor with severe insulation damage, wiring errors, etc., a short circuit may occur. When a short circuit occurs, the current flows through an abnormal path. The instantaneous short-circuit current may reach several times or even hundreds of times the rated current of the motor. If the power supply is not cut off quickly, it may cause serious insulation damage, wire melting, arcing and even causing In the event of a fire, a large electromagnetic force may be generated in the motor to cause irreparable deformation of the winding or mechanical component. Short-circuit protection should meet the requirements: First, it should have instantaneous action characteristics, that is, the power must be cut off within a short period of time; second, when the motor is normally started and braked, the protection device should not malfunction.
Commonly used short circuit protection electrical components are fuses and circuit breakers.
1. Fuse protection. The fuse of the fuse is connected in series in the protected circuit, and is automatically blown when the circuit is short-circuited or severely overloaded, thereby cutting off the circuit and protecting it. Since the fuse melt is affected by many factors, its action value is not stable, so it is suitable for systems with poor motion accuracy and automation, such as small-capacity squirrel cage motors and general AC power supplies.
2. Overcurrent relay protection or circuit breaker protection. The overcurrent relay is a measuring component and the overcurrent protection is done by the actuator contactor. Therefore, in order to cut off the short circuit current, the capacity of the contactor contact has to be increased. The circuit breaker puts the measuring component and the actuator together, and has short-circuit, overload and under-voltage protection functions. This switch can automatically cut off the power automatically when the above-mentioned fault occurs in the circuit. After the fault is removed, the circuit breaker can be re-closed. jobs. Three-phase short-circuit protection shall be used in a three-phase three-wire power supply circuit that uses a two-phase four-wire system for the main circuit or a neutral point grounding for the transformer. If the main circuit capacity is small, the fuse in the circuit can be used as the short circuit protection of the control circuit at the same time; if the main circuit capacity is large, the control circuit must be separately set with 7 short circuit protection fuse.